In general, radial ply tires have many advantages over bias ply tires, for instance, good wear resistance, low rolling resistance, low heat build-up, good durability, good cornering property, etc.
They, however, have a drawback that the shoulder portions extending from both the terminals of the tread portion are apt to wear off and, in addition, an irregular wear phenomenon is likely to occur in which the shoulder portions do not uniformly and equally wear over the circumference.
It is well known in service use as well as in laboratory tests that irregular wear or excessive wear occurs mostly when radial ply tires are used, particularly as front wheels (steering wheels) of a car. Such irregular wear of the shoulder portions at an early stage impairs essential performances inherent in the tires to a great extent.
For example, because of the fact that the shoulder portions wear off early but the rubber material is retained in the central portion of the tread, the wear life of the tire is shortened; the shoulder portions thus worn excessively contact the ground, causing vibration and leading to a state of being ready to slip.
It is generally preferred that when irregular wear has occurred at the front wheel tires in this way, the front wheel tires and rear wheel tires in which irregular wear hardly ever occurs, are switched in order to prolong the life of the tires.
This countermeasure is taught to the public by the manual of standard maintenance and is widely practiced. The tire exchange procedure is, however, troublesome and entails much labour and time, so that it is difficult to make drivers conform to the manual. Thus, the general tendency has been towards aggravating irregular wear of the front wheel tires.
In order to solve these disadvantageous problems, various attempts have hitherto been made. For example, shoulder portions have been altered in belt construction; a tread has been constructed so that the rubbers of the central portion and the lateral portions near the shoulders are different from each other in wear resistance.
To achieve this, however, it is necessary to alter greatly the processing machines in the current production line or to change the order of steps in the line, which leads to too large an increase in cost to meet demand. The idea has therefore not been put into practice.
In view of the current situation, this invention has the object of providing a novel radial ply tire in which a device for diminishing irregular wear or excessive wear can be easily created by a slight modification of the current production line. Specifically, an investigation into fundamental causes for the irregular wear occurring in the shoulder portions was conducted, from which a correlation between the contoured shape of carcass plies and the profile of the tread portion after inflation was deduced. Thus, in this invention, a radial ply tire capable of greatly improved irregular wear resistance is provided by relating the tread profile a tire is expected to assume after air inflation with the configuration of the tread wall of a mould used in the vulcanization-moulding step.